2 Answers
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A volume serial number is a serial
number assigned to a disk volume or
tape volume. It originated in 1950s in
mainframe computer operating systems.
In OS/360 line it is
human-configurable, has a maximum
length of six characters, is in
uppercase, must start with a letter,
and identifies a volume to the system
in unique manner. For example "SYSRES"
is often used for a system residence
volume.

In FAT and NTFS file systems, a volume
serial number is a feature used to
determine if a disk is present in a
drive or not, and to detect if it was
exchanged with another one. This
identification system was created by
Microsoft and IBM during their
development of OS/2.

The FAT/NTFS serial number is a 32-bit
number determined by the date on the
current computer at the time of a
disk's formatting. Previously, the
method used to discern whether a disk
was swapped was reading its volume
label. However, it was not required to
be unique or even non-empty. Many
users had not given disks any name.

In extN and several other filesystems, the serial number is a 128-bit randomly generated UUID.

Yup, these days, it is usually used as a component of uniquely identifying a system, mostly for copy-protection systems. For example, when you buy a program, the developer may assign you a serial number based on a hardware key, which includes the volume serial number as part of the its calculation, in order to lock it to your system.
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SynetechFeb 20 '11 at 6:07